During fabrication of semiconductor integrated circuits, semiconductor wafers are subjected to multiple processing steps at different processing equipment. Fabrication facilities generally include automated material handling systems (AMHS) for transporting the wafers between the processing equipment. FIG. 1 shows a conventional wafer fabrication facility 100 including an AMHS 102.
Referring to FIG. 1, in wafer fabrication facility 100, equipment 104 with similar functions is generally clustered in areas 106, which are called process bays or bays. AMHS 102 includes stockers 108 respectively located at one end of process bays 106 and an inter-bay automated transport rail loop provided as an overhead hoist transport (OHT) 110 located between stockers 108. Each of stockers 108 contains a number of vertically-stacked storage bins for storing semiconductor wafers. OHT 110 has a carrier (not shown) for carrying semiconductor wafers. OHT 110 runs in a loop, picks up wafers from stockers 108, and drops off wafers at stockers 108.
Wafers being processed are at the respective equipment 104. When a process is completed on a wafer, an operator or a technician unloads the wafer from equipment 104 of one of bays 106 and sends the wafer to a nearby first one of stockers 108. OHT 110 picks up the wafer from the first one of stockers 108 and transports it to a second one of stockers 108 next to another one of bays 106 where the next process step is to be performed. The wafer stays in the second one of stockers 108 while waiting for the next processing step. Then, an operator or a technician from the second one of stockers 108 picks up the wafer and loads the wafer into the corresponding equipment 104. Once all required processing on a wafer is complete, the wafer is transported by OHT 110 to a destination such as a test facility or a packaging facility. Wafers are contained in containers such as a standard mechanical interface (SMIF) or a front opening unified pod (FOUP). Each time a container is transferred from one place to another, a barcode on the container is scanned and the transfer of the wafers contained therein is recorded in a computer system for operating AMHS 102.
Thus, between two processing steps, wafers are unloaded from a piece of equipment 104, picked up by an operator or a technician, loaded into one of stockers 108, unloaded from that stocker 108, picked up by OHT 110, transported to a next one of stockers 108, loaded into that next stocker 108, unloaded from that next stocker 108, picked up by an operator or a technician, and loaded into a next piece of equipment 104. Because stockers 108 are generally voluminous, the loading and unloading of wafers are time consuming. For example, each loading of a wafer into one of stockers 108 or unloading of a wafer from stocker 108 may take 3-4 minutes. Also, fabrication facilities are expansive, and each process bay 106 may be, for example, 50 meters long. Walking with wafer containers, especially containers of large size wafers such as 12″ wafers, from a piece of equipment 104 in one of bays 106 to the stocker 108 at the end of that bay 106 may also take several minutes. Consequently, the need to access stockers 108 at the respective ends of process bays 106 between processing steps is a bottleneck that constrains any attempt to improve an efficiency in manufacturing semiconductor integrated circuits.